When electrical components are placed on printed circuit boards for attachment of the component leads to traces on the printed circuit board, the attachment is usually by reflow soldering. The leads and the solder surfaces must be wetted with flux in order to provide for reliable soldering. In previous automatic assembly systems, the surface of the printed circuit board, including the traces thereon, was wetted with flux. This wetting of the printed circuit board produced several problems. It is difficult for the pattern recognition system to identify pads on the circuit traces because of the light reflection from the flux coated surfaces, because both the traces and the printed circuit board dielectric surfaces were flux-coated. The coating of the entire printed circuit board also required that the whole solder pad be wetted. The flux penetrated between the pads and components. This resulted in a longer time to clean off the flux after all of the components were placed and soldered on the printed circuit board. In addition, it is difficult to automate the wetting of the printed circuit board with flux, for use in an automated assembly line. It is thus desirable to individually wet the component leads, rather than the entire printed circuit board.
Surface mounted electrical components on printed circuit board are more widely used. More industries are adopting the surface mount technology in which the leads of the electrical components are attached to the pads on a printed circuit board by lying on the pads, and reflow solder provides the attachment.